1. Field of the Invention
The present invention relates to a filament for electron source, and more particularly to a Schottky electron emission source.
2. Description of Related Art
Electron sources are applied to devices such as electron beam lithography systems, electron microscopes, Auger electron spectrometers, and semiconductor inspection systems. In such devices, an electron source provides electrons, which are then guided into an intense, finely focused beam of electrons having energies within a narrow range. To facilitate formation of such a beam, the electron source should emit a large number of electrons within a narrow energy band. The electrons should be emitted from a small surface area on the source into a narrow cone of emission. Electron sources can be characterized by brightness, which is defined as the electron current divided by the real or virtual product of the emission area and the solid angle through which the electrons are emitted.
Electrons are normally prevented from leaving the atoms at the surface of an object by an energy barrier. The amount of energy required to overcome the energy barrier is known as work function of the surface. A thermionic emission electron source relies primarily on heat to provide the energy to overcome the energy barrier and emit electrons. Thermionic emission sources are not sufficiently bright for use in many applications.
Another type of electron source, a cold field emission electron source, operates at room temperature and relies on a strong electric field to facilitate the emission of electrons by tunneling through the energy barrier. A field electron source typically includes a narrow tip at which electrons leave the surface and are ejected into surrounding vacuum. Since a cold field emission electron source is operated at room temperature, it thus has low chromatic aberration and is used as electron source for high-resolution electron beam device. Also, since cold field emission electron sources are operated at room temperature, gas tends to adsorb to the electron emission surface. Therefore, cold field emission sources exhibit instabilities that cause problems in many applications.
Yet another type of electron source is referred to as a Schottky electron emission source or Schottky emitter. Schottky emitters use a coating on a heated emitter tip to reduce its work function. In Schottky emission mode, Schottky emitter uses a combination of heat and electric field to emit electrons, which appears to radiate from a virtual point source within the tip. With changes to the emitter temperature and electric field, the Schottky emitter will emit in other emission modes or combinations of emissions mode. Schottky emitters are very bright, and are more stable and easier to handle than cold field emitters. Because of their performance and reliability benefits, Schottky emitters have become a common electron source for modern electron beam systems.
As shown in FIG. 1, a typical Schottky electron emission source 100 is provided, which includes an emitter 101 having an apex 108 from which the electrons are emitted, a coating 102 on the emitter 101, a tungsten filament 103 for supporting the emitter 101, two electrodes 104 for mounting the filament 103, and a base 105 for supporting the two electrodes 104. The tungsten filament 103 is a bended wire and heated for providing heat to the emitter 101. Schottky electron emission source 100 typically operates with apex 108 at a temperature of approximately 1800K. Emitter 101 is typically made from a single crystal of tungsten oriented in the <100>, <110>, <111>, or <310> orientation. Emitter 101 is coated with a coating material to lower its work function. Such coating materials could include, for example, compounds, such as oxide, nitrides, and carbon compound, of zirconium, titanium, hafnium, yttrium, niobium, vanadium, thorium, scandium, beryllium, or lanthanum. For example, coating a (100) surface of tungsten with zirconium and oxygen lowers the work function of the surface from 4.5 eV to 2.8 eV. By reducing the energy required to emit electrons, the coating on the emitter makes it a brighter electron source.
Conventional thin (0.005 inch=0.127 mm) tungsten filament is used for electron source. However, this structure, thin tungsten filament 103, will induce vibration issue, and this issue can also be referred to U.S. Patent Publication No. 2010/0090581. A simple way to depress the vibration issue is to thicken tungsten filament directly. Nevertheless, the tungsten filament 103 cannot be thickened directly, because low resistance of tungsten will incur higher operation current larger than 3 A(ampere). Power supply can hardly provide such a large current for electric circuit of the electron source 100.
Two companies, KIMBALL PHYSICS and DENKA, provide a ribbon-type tungsten filament 203 used for electron source 200 to resolve the vibration issue, as shown in FIG. 2. Although this ribbon-type structure may inhibit the vibration issue, it will make the tungsten filament 203 not easily be welded to electrodes 204, because the ribbon-type filament 203 with a certain width-to-thickness ratio will cause welding issue while small side of the ribbon-type filament 203 welds on the electrodes 204.
Another solution is provided by DENKA, as shown in FIG. 3 and also referred to U.S. Patent Publication No. 2010/0090581. A cup-shaped component (6) is added to support the tip (1), and a tungsten filament (3) is used to heat up the cup-shaped component (6). Thus, the vibration issue is also resolved. However, cup-shaped tungsten component with tungsten filament assembly has complex structure (each component is very tiny, about 0.1-0.01 mm), and no commercial product available now. Further, the cup-shaped component (6) is heated up indirectly by the tungsten filament (3), i.e., not electrothermal means to heat up the cup-shaped component (6). Such a heating process has lower thermal efficiency, and will incur more power consumption to achieve the same performance
Accordingly, a new filament should be provided not only to solve the vibration issue, but also to keep all advantages that conventional tungsten filament possesses.